Two main approaches: one - use a set of flaws to increase the available points and spend these on another / a bunch of linked attack vectors (e.g. like disintegrate does with drain then damage). Thus the attack can be more effective even though it is still PL capped. The other approach is the often forgotten use of an HP to up an attack PL, which can breach caps.

In this example, there are a some headscratchers though: as the rifle is already easily removable, then there shouldn't be many additional points to be gained from the power loss aspect. As a I recall, the star trek overload was both very slow (longer than one turn), required a good bit of fiddling to start (i.e. a full action to start the overload process) and had a big obvious whine that rather gave the game away that an explosion was imminent. Those would be aspects that could certainly provide flaws that would provide points that would then feed into linked attack vectors.

Personally, I'd work the whole thing out and then not actually buy it: agree it with the GM as a pre-built stunt and then use an HP to stunt it in the very unusual case you'd need/want to use it, possibly burning more than one HP if you want to up the PL of the stunted attack even higher.

Doing it as a power stunt is actually not a bad idea. I hadn't thought of that.

I could also approach it from the opposite direction: build the gun-destroying explosion the main power, with the normal Blast 8 as the AP instead.

And this assumes that this particular capability of the gun even needs to be converted. IIRC, the player very rarely ever used it. In M&M, it could be more of a plot device to be used only when I say it would be ok.

Organic material taken form the core of Halley's Comet in 1987, seeded into a humanoid form and given sentience: she is THE COMET!

Coming into this late, but there is a rule for Extra Effort targeting the device instead of the player. Fatigued Devices lose 1 rank each on all of their effects. I forget what Exhausted does. Use of the "+2 ranks for the attack" function of Extra Effort and possible Extraordinary Effort would have a rifle that temporarily exceeds PL, but at the cost of requiring being rebuilt afterwards.